Recently, in an engine applied to a vehicle such as a diesel engine, a urea selective catalytic reduction system (urea SCR system) is developed and produced as an exhaust purification system that purifies nitrogen oxides (NOx) in an exhaust gas with a high purification rate.
The urea SCR system includes a pump that pumps a urea water stored in a tank as a reducing agent to a reducing-agent passage, and a injector that injects the urea water pumped through the reducing-agent passage into an exhaust pipe of the engine.
In the urea SCR system, the exhaust gas is purified by a reduction reaction of NOx at a NOx purification catalyst that is a SCR catalyst in the exhaust pipe. In the reduction reaction of NOx, the urea water injected from the injector into the exhaust pipe is hydrolyzed by an exhaust-gas heat to generate ammonia (NH3), and the ammonia is adsorbed at the SCR catalyst. The NOx in the exhaust gas is reduced at the SCR catalyst by a reduction reaction executed by the ammonia, to be purified.
In the urea SCR system, air may enter the reducing-agent passage. In other words, an air mixing may exist in the reducing-agent passage. For example, in the urea SCR system, a draw and return processing is executed to draw and return the urea water in the reducing-agent passage to the tank to maintain the urea water not to be frozen in the reducing-agent passage when the engine is stopped. Thus, in the urea SCR system, the reducing-agent passage is filled with the urea water when the engine starts. In this case, the air may enter the reducing-agent passage. When the air enters the reducing-agent passage, an injection quantity of the urea water injected from the injector to the exhaust pipe becomes unstable.
JP5338696B2 discloses a technology to detect whether the air enters a fuel passage from a fuel pressure in the fuel passage. By applying the technology to the urea SCR system, it can be determined whether the air mixing exists in the reducing-agent passage from a pressure of the urea water in the reducing-agent passage.